An IOT device includes an electron gun arranged to produce a linear electron beam and an input resonant cavity at which an r.f. signal to be amplified is applied to produce modulation of the beam at a grid of the electron gun. The resultant interaction between the r.f. energy and the electron beam causes amplification of the high frequency signal which is then extracted from an output resonant cavity circuit.
One problem faced by designers and operators of IOT devices is that the dimensions of the arrangement, particularly the resonator cavities included at the input and output parts of the device, must be precisely chosen for a particular band of operating frequencies. The resonant frequency of resonant cavities may be altered using tuning doors, or movable tuning plates, to change their volume. However coupling efficiency between primary and secondary resonant cavities may be adversely affected when operated at the extremes of the frequency band, even to the extent that it may be impracticable to use the device. In such cases, a second tube suitable for use over a different range of frequencies may be required.
The present invention arose from the particular consideration of the output cavity arrangement of IOT devices but it is also applicable to other electron beam tube arrangements in which coupling between resonant cavities is employed, such as, for example, klystrons.